The present invention relates to a binary transversal filter and, more specifically, to a binary transversal filter which is principally used in digital radio-relay systems.
Digital radio-relay systems are dependent upon a complicated transmission system, such as cosine roll-off filtering or the like, in order to effectively utilize the frequency and to reduce interference among codes being transmitted. In the above mentioned transmission system, a binary transversal filter made up of digital integrated circuits is employed.
Disclosure of a conventional binary transversal filter can be found, for example, in IEEE Transactions on Communication Technology, Vol. COM-16, No. 1, February, 1968 pp. 81-93, Herbert B. Voelcker: "Generation of Digital Signaling Waveforms". According to the binary transversal filter disclosed in this article, when it is desired to drive a plurality of stages of shift registers by timing signals to a frequency which is M times greater than that of the clock pulses of the NRZ (non-return-to-zero) signals being introduced, the introduced NRZ signals are converted into RZ (return-to-zero) signals of a pulse width T/M (wherein T denotes a repetitive period of the input NRZ signals) and are fed to the shift registers, and the outputs of the shift registers of each of the stages are synthesized to produce output signals. A tap coefficient for weighting the outputs of shift registers of each of the stages can be found based upon a sampling value at a time of an interval T/M of impulse response of the filter. For example, if it is assumed that M=2, the tap coefficient can be obtained by ##EQU1## where a(t) denotes an impulse response of a transfer function H(.omega.) which is being found.
The tap coefficient given by the above equation (1), however, only holds true when the signals are introduced in the form of impulses. Actually, rectangular pulses of a width of T/2 (M=2) are introduced, and, therefore, the tap coefficient a.sub.n ' is given by ##EQU2##
According to the above-mentioned conventional techniques in which the input NRZ signals are converted into RZ signals and are fed to the shift registers, however, the RZ signals thus converted contain frequency components of the clock pulses giving rise to the occurrence of a spike carrier corresponding to the frequency spectrum of the clock pulses, and the frequency components corresponding to this frequency spectrum are radiated into space as spurious components which fall outside the transmission band. With the conventional technique, therefore, other communications circuits relying upon the above-mentioned frequency components were often disturbed. In addition, a converter for converting the NRZ signals into RZ signals tends to become more complicated in construction with an increase in the multiplier M.